Transcript Transition Metal Complexes of Topologically Constrained

Transition Metal Complexes of
Topologically Constrained
Tetraazamacrocycles
Tim Hubin
Chemistry Department
University of Kansas
Motivation: Aqueous Oxidation Catalysts
 Consumer
Product Applications (Procter & Gamble)
– Laundry Detergents
– Hard-Surface Cleaners
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H2O2 oxidation of soils and stains
 Industrial Applications
– Pulp Bleaching
– Synthetic Organic Oxidations
 Primary Advantages
– Environmentally Friendly Solvent/Oxidants
– Inexpensive Solvent
Challenges to Aqueous Oxidations
 Solubility
– Applications require aqueous/organic solubility
– Water soluble substrates or biphasic reactions
 Catalyst
Decomposition
– Transition Metal Complexes decompose in H+ or OH» Acidic Conditions
R3N
M
H+
R3NH+
+
M
» Basic Conditions
R3N
M
OH-
R3N
+
M(OH)n
» Oxygenated Conditions
R3N
M
O2/H2O
R3N
+
MxOy (Examples: MnO2 or Fe2O3)
Overall Project Goals
 Design
and Synthesize Ligands to stabilize transition
metal ions in harsh aqueous conditions
 Synthesize and Characterize transition metal
complexes of the ligands (Mn and Fe preferred)
 Evaluate complexes as oxidation catalysts in pH 10
water with H2O2 or O2 as oxidant
 Determine mechanism(s) and preferred substrates of
active catalysts
 Redesign Ligands for improved activity
Metal-Ligand Binding Affinity
 Complementarity:
match between metal and ligand
leading to molecular recognition (minimum for strong
binding)
– size
– geometry
– electronics
O
O
O
K+
O
O
O
 Constraint:
factors reducing freedom in ligand
systems and leading to optimization of binding
affinity if complementarity is maintained
– topology--connectedness of donor atoms in a ligand
– rigidity--inflexibility or fixedness of donor atoms in a
ligand
Topological and Rigidity Effects
NH2
NH
NH3
NH
HN
NH
HN HN
NH
HN
NH
HN HN
HN
NH2
NH2 H2N
Increasing Topological Contraint and Complex Stability
H2N
NH2
N
N
N
Increasing Rigidity and Complex Stability
N
Cross-Bridged Tetraazamacrocycles



CH3
H3C
N
N
N
N




Topologically constrained like a cryptate
Short cross-bridge rigidifies the macrocycle
Tunable: ring size and Me group can be
modified
Simple, high yielding organic synthesis
Leaves octahedral metal ions coordinatively
unsaturated
Neutral ligand giving charged complexes
Resistant to oxidation
– Tertiary amines
– Saturated
Secondary Goals
 Understand
ligand synthesis
 Apply synthesis to other macrocycles
 Explore coordination chemistry of rigid intermediates
 Quantify solution behavior of ligands
– Proton Sponges
 Overcome
proton sponge problem to develop
coordination chemistry of cross-bridged ligands
 Fully characterize complexes--structure,
spectroscopy, electrochemistry, solution behavior
Ligand Synthesis
O
n
NH HN
O
H
H
CH3CN
NH HN
N
N
H
H
N
N+
R
H
N
RX
n = 0 or 1 independently
RX = MeI or BnBr
CH3CN
N
n
n
H
n
n
n
R
N+
N
NaBH4
2X
-
N
N
N
N
95% EtOH
R
if R = Bn
Pd/C, H2
n
N
HN
HOAc
NH
N
R
n
n
Reference: Weisman, G. R.; Wong, E. H.; Hill, D. C.; Rogers, M. E.; Reed, D. P.;
Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1996, 947.
n
Ligands and Abbreviations
N
N
N
N
CH3
N
N
N
N
N
NH
HN
N
CH3
Me2(B14N4)
N
N
N
N
Bn2(B14N4)
H2(B14N4)
CH3
N
N
N
N
N
NH
HN
N
CH3
Me2(B12N4)
Bn2(B12N4)
N
N
N
N
CH3
H2(B12N4)
N
N
N
N
CH3
CH3
CH3
Me2(B13N4)
Me2(B14N4Me6)
Me2(B14N4) Synthesis
H2(Q14N4)2+
Me(Q14N4)+
HMe(Q14N4)2+
Me2(Q14N4)2+
H2Me2(B14N4)2+
Coordination Chemistry of Q#N4
 Tetracyclic
 PdII,
intermediates not yet used as ligands
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
CuI, and CuII can coordinate
Cu(Q12N4)Cl2
Cu(Q13N4)Cl2
Cu(Q14N4)Cl2
Cu(iso-Q14N4)Cl2
Solution Behavior of Cross-Bridged Ligands
 Potentiometric
titrations confirm proton sponge
behavior
– B14N4: pKa2 = 9.58, pKa1 = ?
– B14N4Me6: pKa2 = 11.45, pKa1 = ?
12
11
10
9
8
pH
Observed
7
Calculated
6
5
4
H2Me2(B14N4Me6
)2+
3
2
-2.15
-1.15
-0.15
0.85
Equivalents Base
1.85
2.85
3.85
Overcoming the Proton Sponge Problem
n
N
+
NH
R
HN
+
N
n
1. Extraction from
pH = 14 water
N
2. Vacuum distillation
from KOH
N
R
R
N
N
MSaX2
N
anhydrous, anaerobic,
aprotic solvent
R
R
n
n
Proton Sponges
Free Ligands
N
X
M
X
N
N
R
Transition Metal
Complexes
Metal Complexes:
Macrocycle Rings
12, 13, 14, 14Me6
R-groups
Me, Bn, H
J. Chem. Soc., Chem. Commun., 1998, 1675.
Metals
Cr, Mn, Fe, Co
Ni, Cu, Zn, Pd
Crystallographic Characterization
Co(Me2B12N4)Cl2
[Ni(Me2B14N4)(acac)]+
Fe(Bn2B12N4)Cl2
Me2B14N4Me6 Forces Pentacoordination
Co(Me2B14N4Me6)Cl+
Kinetic Stability of Complexes
 Basic Conditions (1 M KOH)
– FeII/MnII complexes--several days for any oxides to form
– MnII(Me2B14N4)Cl2--isolated MnIII intact complex
 Acidic
Conditions (1 M HClO4)
Metal
MnII
Ligand
Me2B14N4
Me2B12N4
t1/2
13.8 h
< 15 min
ZnII
Me2B14N4
Me2B13N4
Me2B12N4
3.9 h
0.3 h
0.8 h
CuII
Me2B14N4Me6
Me2B14N4
Me2B13N4
Me2B12N4
> 8 yr
> 6 yr
>8 yr
30 h
Metal
MnII
Ligand
porph
14N4Me6
t1/2
74 x 10-6 s
<1s
CuII
Me414N4
2s
cis-14N4Me6
2s
trans-14N4Me6 22 d
Electrochemical Studies
 Ligands
stabilize metal
in multiple ox. states
Cyclic Voltammetry of Me2B14N4 Complexes
CuII
NiII
 Ring-size
effect--larger
ring increases ox. pot.
 N-Substituent
CoII
effect--
FeII
– R = Me, Bn approx. same
– R = H lowers ox. pot.
2
MnII
1.5
1
0.5
0
-0.5
Potential (V) vs SHE
-1
-1.5
-2
-2.5
Electronic Spectra--Ligand Field Strength

NiII complexes allow
approximation of Do

Cross-bridged ligands do
not have stronger M-N
bonds than simple
macrocycles

Observed kinetic stability
likely comes from
topological constraint and
rigidity, not strong M-N
bonds
0.01 M Ni(Bcyclen)Cl2 in DMF
Absorbance
0.4
0.3
0.2
0.1
0
300
500
700
900
1100
Wavelength (nm)
Octahedral approximation
Ni(Me2(B14N4))Cl2 Do = 10,215 cm-1
Ni(Me2(B12N4))Cl2 Do = 9,843 cm-1
Ni(cis-13N4)Cl2
Do = 11,110 cm-1
Ni(trans-14N4)Cl2
Dqxy = 14,870 cm-1
Oxidation Catalysis
MII complexes were screened initially on the
blue dye “Direct Blue 1” pH 10, H2O2 oxidant
 Several
NH2 OH
NaO3S
OH
N
N
N
CH3O
SO3Na
N
SO 3Na
???
Pink>>>>Colorless
OCH3
SO3Na
Deep Blue
MnII best metal
 Substrate
NH2
Me2(B14N4) best ligand
Oxidations (Dr. Maria Buchalova)
– Alkene Epoxidation--poor catalyst
catalyst
H2O2
O
– H-atom Abstraction--effective catalyst
catalyst
H2O2
Patent Applications: WO 98/39098
WO 98/39406
Higher Valent Complexes
 Oxidation
Mechanism(s), while still under
investigation, likely involve higher oxidation states
– MnIII Complexes
PF6-
Mn(Me2(B14N4))Cl2 + Br2
[Mn(Me2(B14N4))Cl2]PF6
MeOH
11.0
10.0
+1.343(76)
9.0
8.0
pKa2 = 5.87(2)
p[H+]
7.0
6.0
5.0
4.0
+0.582(72)
pKa1 = 1.6(2)
3.0
2.0
-4.00
1.600
-3.00
-2.00
-1.00
0.00
Equivalents Base
1.00
2.00
3.00
1.400
1.200
1.000
0.800
0.600
Potential (V)
0.400
0.200
0.000
Dimerization of High Valent Mn
 Oxidized
synthetic Mn complexes often dimerize
with m-oxo or m-hydroxo bridges
OH2
HN
Mn
oxidant
OH2
HN
NH
NH
NH
O
HN
Mn
Mn
O
HN
NH
NH
NH
NH
NH
MnIII/MnIV dimer
has characteristic
16-line EPR spectra,
green color
 For
cross-bridged ligands: No dimers if R = Bn or
Me, but green complex in air if R = H
Mn(H2(B12N4))Cl2 + Air
1500
2500
3500
4500
5500
MnIII Complexes
 Attempted
Dimerization
Mn(Me2(B14N4))Cl2 +
OH-
PF6-
[Mn(Me2(B14N4))(OAc)(OH)]PF6
H2O/EtOH
+0.505 (62)
-0.689(132)
Mn-OH = 1.812(4)
(1.816(4) and 1.827(3) literature)
IR: 3649 cm-1 (M-OH)
0.800
0.600
0.400
0.200
0.000
-0.200
Potential (V)
-0.400
-0.600
-0.800
-1.000
Summary

Overall Goals
4 Design and synthesize Ligands
4 Synthesize and characterize
transition metal complexes
4 Evaluate complexes as oxidation
catalysts
* Determine mechanism(s)
* Redesign Ligands

Secondary Goals
4 Understand ligand synthesis
4 Apply it to other macrocycles
4 Explore coordination chemistry of
rigid intermediates
(4) Quantify solution behavior of
ligands (Proton Sponges)
4 Overcome proton sponge problem
to develop coordination chemistry
of cross-bridged ligands
(4) Fully characterize complexes-structure, spectroscopy,
electrochemistry, solution
behavior
Acknowledgments
Prof. Daryle H. Busch and Busch Research Group
Aqueous Oxidation Subgroup
Synthesis:
Dr. Simon Collinson
Tim Hubin
Dr. Jim McCormick
Dr. Nickolay Tyryshkin
Collaborators
Dr. Chris Perkins (P & G)
George Hiler (P & G)
Physical Methods: Dr. Jim McCormick
Tim Hubin
Dr. Nathaniel W. Alcock (Warwick)
Dr. Howard J. Clase (Warwick)
Dr. Pawan K. Kahol (WSU)
Dr. Ahasuya Raghunathan (WSU)
Dr. Martha Morton (KU)
Mechanism:
Dr. Regine Labeque (P & G)
Dr. Maria Buchalova
Dr. Jim McCormick
$$$
Procter & Gamble
Madison A. and Lila Graduate Fellowship of KU
Topological Effects: chelate to cryptate
 Increased
M/L Affinity with increased topological
constraint
NH2
NH
NH3
NH
HN
NH
HN HN
NH
HN
NH
HN HN
HN
NH2
NH2 H2N
 Thermodynamic
Causes
– Entropy Increased
M
H2N
H2N
NH3
+
M
+
NH3
H2N
2 NH3
H2N
– N affinity for M+-- RNH2 binds more strongly than NH3
More Topological Effects

M
Thermodynamic Causes
– Preorganization--less motion
needed for a metal to ligate a
macrocycle than a linear
polyamine

H2N
NH HN
N
H
NH2
N
H
M
NH HN
Kinetic Causes
– High Effective Concentration
=> fast ring closure
– For Macrocycles and
Cryptates, slow ring opening
is due to deformations
required (no end group)
N
N
M
fast
N
M
N
N
N
N
N
M
N
N
N
M
N
N
M
N
N
N
Rigidity Effects
 A rigid,
preorganized structure requires little
rearrangement to bind a metal ion (thermodynamic)
 More
rigid structures make deformations needed for
stepwise dissociation more difficult (kinetic)
H2N
NH2
N
N
N
Increasing Rigidity and Complex Stability
N
Considerations for Catalytic Systems
 Open
Coordination Site(s) on M for oxidant/substrate
binding
 Multiple oxidation states of M accessible, but not too
stable for any single oxidation state
 Ligand must be resistant to oxidation
 Resulting Complexes should be water soluble
– Neutral ligand (cationic complex)
– Lack of “greasy” groups
 Organic
Solubility desirable
– Biphasic oxidations of organic molecules
– Surface cleaning of organic molecules
Probing the Ligand Synthesis
NH HN
N
NH HN
N
H
H
N
N
N
N
H
H
N
+
CH3
+
CH3
N
meso-14N4Me6
NH HN
N
NH HN
N
racemic-14N4Me6
H
H
N
N
N
N
H
+
H3C
H
N
N
H3C
N
N
N
N
CH3
Crystallography of B14N4 Synthesis
H
CH3
CH3
N
N+
N
N
N
N+
N
N
N
N
N+
CH3
H
N
N+
N
N+
N+
N
N+
N
(+)
N
X
H
H2(Q14N4)2+
Q14N4
H3C
Me(Q14N4)+ {X = :}
HMe(Q14N4)2+ {X = H}
H
H3C
Me2(Q14N4)2+
H2Me2(B14N4)2+
CH3
CH3
CH3
H
N
N
N
N
N
N+
N
N+
N
N+
N
N
N
N
N
N+
N+
N
N+
N
H
meso-Q14N4Me6 racemic-Q14N4Me6
HMe(Q14N4Me6)2+
H3C
Me2(Q14N4Me6)2+
H3C
H
H2Me2(B14N4Me6)2+
racemic-Me2(B14N4Me6) Synthesis
Q14N4Me6
HMe(Q14N4Me6)2+
Me2(Q14N4Me6)2+
H2Me2(B14N4Me6)2+
Why Not meso-Me2(B14N4Me6)?
racemic-Q14N4Me6
meso-Q14N4Me6
MnIII Complexes (cont’d)
Mn(Me2(B14N4))Cl2 + OH-
PF6-
{ [Mn(Me (B14N4))(OH) ]PF }
2
2
6
H2O/EtOH
MeOH
[Mn(Me2(B14N4))(OMe)2]PF6
+0.542(243)
-0.781(628)
1.500
1.000
0.500
0.000
-0.500
Potential (V)
-1.000
-1.500
-2.000
Kinetic Stability of Complexes
 Basic
Conditions (1 M KOH)
– FeII/MnII complexes--several days for any oxides to form
– MnII(Me2(B14N4))Cl2--isolated MnIII intact complex

Acidic Conditions (1 M HClO4)
–
–
–
–
–
Mn(Me2(B14N4))Cl2--t1/2 = 13.8 h (2.3 x 10-8 s)
Zn(Me2(B14N4))Cl2--t1/2 = 3.9 h (2.3 x 10-8 s)
Zn(Me2(B12N4))Cl2--t1/2 = 0.83 h
[Cu(Me2(B14N4))Cl]Cl--t1/2 > 6 years at 40 oC (1.4 x 10-9s)
[Cu(Me2(B14N4Me6))Cl]Cl-- t1/2 > 8 years at 40 oC
» Cu(Me414N4)2+-- t1/2 = 2.0 s in 1 M HNO3
» Cu(cis-14N4Me6)2+-- t1/2 = 2.0 s in 6.1 M HCl
» Cu(trans-14N4Me6)2+-- t1/2 = 22 days in 6.1 M HCl
Crystallographic Characterization (cont’d)
[Cu(Bn2B14N4)]+
[Cu(Me2(B14N4Me6))Cl]+
[Pd(Me2B14N4)Cl]+
Electron Configuration

Magnetic Moments

– FeII, MnII, NiII complexes are
high spin, as Do predicts
EPR Studies
– MnII complexes show typical
high spin d5 transition split by
I = 5/2 55Mn nuclei
(a)
(b)
2500
2700
2900
3100
3300
3500
3700
Magnetic Field (G)
meff = 5.80 ± 0.06 BM for Mn(Me2(B14N4))Cl2
a = Mn(Me2(B12N4))Cl2
b = Mn(Me2(B14N4))Cl2
3900
FeIII Complexes
– Fe(Me2(B#N4))Cl2 + Br2
FeIII EPR signal
X-Band EPR Spectrum of Fe(B13N4)Cl2+
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Gauss
– Fe(H2B#N4)Cl2 + Air
[Cl(H2(B12N4))Fe-O-Fe(H2(B12N4))Cl]Cl2